why not just make your own game at this point for some extra coin?
There are several reasons. It's not only a matter of "some extra coins".
Quest system/Turn based system/Editor&Tools/Dialogue system/Scripting system/Resource & Memory management/etc... etc.. etc...
They are going to be a pain in the ass to be coded from scratch.
I mean, Larian Studios is a SH with something like 100 or more employee I guess. Redoing an engine/framework for a game similar to D:OS with your custom rules and assets sounds a bit odd to me.
